1. Field of the Invention
The present invention relates to a method for fabricating a preform for molded lenses, and a preform fabricated by that method.
2. Description of Related Art
It was in 1987 that aspheric lens production was initiated by molding rather than by polishing.
According to one typical method for making such aspheric lenses, lens preforms having a surface roughness of up to 100 .ANG. are pressed in a mold at a temperature lying within the viscous range of 10.sup.8 to 10.sup.12 poises for a period of at least 20 seconds, while the temperature of the glass is kept equal to the temperature of the mold (see JP-B 56-378).
The mold used must then be built up of material that does not fuse to glass. For instance, SiC with a carbon coat on it, Al.sub.2 O.sub.3 with a chromium oxide coat on it and the like are used (see M. KOBAYASHI and T. OSHITA, "Tungalloy", 32, 43 (1992)).
It is here noted that the preforms for molded lenses are in (1) spherical, and (2) virtually oval forms, because they must be pressed at the center position of a lens mold. However,
1) the preforms should not only have a surface roughness equal to a polishing roughens of up to 100 .ANG.), but also be free from surface defects such as shear marks, and PA1 2) the preforms should have their weight variation limited to up to 0.5%, because their weight has a close relation to the final lens thickness. PA1 1) the glass used should have surface tension high-enough to form the glass droplet 3 while it is well balanced between weight and surface tension. PA1 2) Too large pipe diameter makes it difficult to form any spherical droplet having a certain or larger diameter (e.g., larger than 10 mm). Thus, the prior method is unsuitable for making a complete preform. PA1 1. A method for fabricating a preform for molded lenses, in which glass is formed into a spherical or oval preform by fire-polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises) of the glass. PA1 2. A method for fabricating a preform for molded lenses, in which glass is formed into a spherical or oval preform by firing polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises) of the glass, preferably at the softening point (the viscosity of 10.sup.7.6 poises) of the glass. PA1 3. A spherical or oval preform for molded lenses, which is formed by fire polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises). PA1 4. A method for fabricating a preform for molded lenses, in which a polygonal or substantially spherical form of glass is placed on a heating dish built up of material that is not fused to the glass, and is then heated for fire polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises). PA1 5. A preform for molded lenses, which is fabricated by placing a polygonal or substantially spherical form of glass on a heating dish built up of material that is not fused to the glass, and heating the glass for fire polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises). PA1 6. A method for fabricating a preform for molded lenses, in which a die-like or spherical form of glass is placed on a heating dish made up of material that is not fused to the glass, and is then heated for fire polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises) for a period of time of 10 minutes or more. PA1 7. A preform for molded lenses, which is fabricated by placing a die-like or spherical form of glass on a heating dish built up of material that is not fused to the glass, and heating the glass for fire polishing at a temperature between the flow-point (the viscosity of 10.sup.5 poises) and yield point (the viscosity of 10.sup.10 poises) for a period of time of 10 minutes or more.
To provide optical surface roughness and a constant weight to preforms for molded lenses, they may possibly be made by a method for making ordinary spherical lenses by polishing. However, this method is not economical in such a sense that molded lenses cost twice as much as polished lenses.
To solve this problem, those skilled in the art have come up with such a method as illustrated in FIG. 1. According to this method, a glass droplet 3 is formed at the distal end of a pipe 2 attached to the bottom of a crucible 1. Then, that distal end is cut 9 so that the glass droplet 3 can drop down from the lower end of the pipe 2 in a substantially spherical form. This droplet 3 receives hot air 5 injected out of a funnel pipe 4, so that it can be cooled without coming into contact with any object, thereby making a preform with no surface defect.
However, this prior method has some problems. That is,
Hence, the material used with the prior method is limited to glass species having high surface tension, e.g., SK and LaF.
In view of such problems as mentioned above in connection with the prior art, an object of the invention is to provide a method for fabricating a preform for molded lenses, which is not limited by the surface tension of glass and so is applicable to every glass, and is not limited by glass size at all. Another object of the invention is to provide a preform fabricated by such a method.
The above and other objects and features are achieved by the invention set forth hereinafter.